[Accessibility]: Remove aria-hidden from ToolbarInput and set label
[Accessibility]: Remove aria-hidden from ToolbarInput and set label

Issue:
- The ToolbarInput text prompt is marked as aria-hidden which is causing trouble for screen readers such as voiceover

Changes:
- Remove markAsHidden call from ToolbarInput
- Setting the prompt element's label to the placeholder string
    - The original reason to mark the element as hidden was because Accessibility insights flagged that this input element did not have a label
    - See original change introducing markAsHidden call: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1987841

Bug: 1194771
Change-Id: Icb2be92f3a45077ad7238656e0c80a0418477d85
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2798583
Commit-Queue: Michael Liao <00d75d81082fdf74688ec453c516b42f7cbf0f40@microsoft.com>
Reviewed-by: Brandon Walderman <5d029aa09a451e20f81a28513ba454595225585f@microsoft.com>
Reviewed-by: Alex Rudenko <d8d88e7d7e77f69e23d1d1432231196fb8e02a62@chromium.org>

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